1. Field of the Invention
This invention lies in the field of electronic sensors of bioelectrical potentials, and more particularly relates to an improved electronic device for collecting and selecting the biopotentials existing between the cell and the medium outside the nucleus membrane as a consequence of the metabolical activity of the cell under study.
1. Description of the Prior Art
Systems for detecting cell biopotentials are conventionally characterized by a microelectrode introduced into the nucleoplasma or cytoplasma and another electrode which is placed at random in the close neighborhood outside the nuclear or cell membrane. The bioelectrical resting or action potentials supplied by the metabolical activity of the cell are led by means of microelectrodes and connections to an electronic device for adaptation and amplification, as well as to processing and record circuits.
The requirements for intracell measurements are very severe; some sensitive neurons may tolerate without destroying a straight flow current of some tenths of picoamperes, so that the input flow of current to the preamplifier may, in the worst case, be a maximum of 0.5 picoamperes. The value of the resting potential of the cell membrane is a maximum of 100 milli Volts (mV) in most cases (Bures, J. et al. Electrophysiological methods in biological research, Prague 1967), and the resting potentials of the nuclear membrane is about -12 mV(Loewenstein, W. R. and Kanno, Y., Nature, 1962, 195:462-464); the finite input impedance of the measurement amplifier, which is the load of the cell, must be at least 100 GigaOhms for causing a flowing stream under 0.12 pAmps. The action potentials are often measured; they include some rapid signals like a sharp impulse whose rise time is of 50-100 microseconds and having a duration of 0.2-15 milliseconds. For transmitting such impulses a considerable decrease of the capacity of the cable to the preamplifier and its input capacity is necessary.
When a metal microelectrode is introduced into the cell, the potential appearing between the microelectrode terminal and reference microelectrode is the sum of three potentials, (1) the contact metal-electrolyte potential, (2) the membrane cell potential, and (3) the contact potential between the reference electrode and the extracell surroundings.
When the membrane potential is measured, the sum of the other two terms is assured to be constant (Electrodes and the measurement of bioelectric events, by Geddes, L. A., Wiley-Interscience, 1972).
When the microelectrode is introduced into the cell nucleus, the output potential is made up of four potentials: (1) the contact metal-nuclear electrolyte potential, (2) the nucleus membrane potential, (3) the cell membrane potential, and (4) the contact potential between the reference electrode and the extracell space. See "Some electrical properties of a nuclear membrane examined with a microelectrode," W. R. Loewenstein and Kanno, "The Journal of General Physiology, Vol. 46, 1963."
The potential generated at the level of nuclear membrane has a smaller amplitude and time constant than in the cell membrane. As a result, the action potential due to nucleus depolarization has a shorter duration that the one released at the cell membrane level, (Lowenenstein and Kanno, 1962-1963).
In pursuit of the objective of a biopotential preamplifier having a high impedance with an effective reduction of its input capacity and having the possibility of automatic compensation of direct current levels which drain in the amplification chain, including the rest potential of the cell, an electronic device has been achieved containing a commercial cathode follower with input capacity neutralizing and a circuit for the automatic compensation of the direct current level. See: "An Automatic D.C.-Level Compensation Circuit for Electrophysiology," G. I. Allen and K. Toyama, "IEEE Transactions of Biomedical Engineering, Vol. 20, Jan. 1973."
Although commercial preamplifiers with unitary gain of the cathode follower type, as (MPA-5 Transidyne General, Ann Arbor, Michigan; Fantron Amplifier Mod. 5791, Basel, Switzerland; Microelectrode Preamplifier Type 3111; Narco Bio-Systems Inc., Houston, Tex., etc.) offer a high input impedance and a substantial reduction of input capacity, by manual neutralization, nevertheless the noise at the amplifier non inverse input due to positive capacity feedback remains in the range of 30-70 micro Volts rms.
By using an automatic compensation device for D.C. level (Allen and Toyama, 1973) in measuring the nuclear potentials, some valuable informations can remain unobserved due to integration with a fixed constant time, as well as its inversion circuit.
A considerable increase of the signal-noise ratio and accuracy of unitary amplification of biopotentials, both nuclear and cellular, may be obtained by a more accentuated reduction of the noise produced at the preamplifier input, as well as by creating a controlled selection of certain action potentials according to the electrical parameters of the two membranes: nuclear and cellular, implied in their production.